ARM: dts: msm: Enable GPU driver SMMU retention flag for SDM845

GPU SMMU driver already supports retention. Enabling retention flag in
GPU driver removes the need to attach/detach pagetable to SMMU context bank
every GPU power collapse cycle. This helps wakeup latency and power.

Change-Id: Ib6fccf1b6ee0dd654a363a9f365f48b8cc985ab0
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>